You have already studied the ANSI Standard form of C programs. Unfortunately most books on X programming still use the original Kernighan and Ritchie (K &R) C.
This is because some of X is still written in this style.
The major differences between K &R and ANSI C that concern us are:
void swap(a,b) AND NOT void swap(int *a, *b)
int *a, *b;
We can (and should) still write our X window programs in ANSI C.
There is a c89 compiler option -common that we will have to use to tell the compiler to understand K &R syntax as well and turn off prototyping. (See later for more details on compling X window programs.
Alternatively you can write totally in K &R C and use the cc compiler.